After manufacture of a semiconductor memory array chip is complete, integrated circuit (IC) patterns on an exposed surface of the chip are sealed with an electrically insulating layer of passivating material. Typical passivating materials include resins or thermoplastic polymers such as, for example, polyimide. The purpose of this final “passivation” layer is to prevent the surface of the chip from reacting chemically with ambient moisture, to protect the surface from environmental particulates, and to absorb mechanical stress. Following passivation, the chip is mounted in an electronic package embedded with metal interconnects that allow probing and functional testing of the memory cells. When one of many redundant memory cells is determined to be faulty, the cell is disabled by severing the conductive interconnects, or wires, linking that cell to its neighbors in the array. Disabling individual memory cells by “link processing” or “link blowing” is accomplished by laser micromachining equipment that is capable of directing laser beam energy so as to selectively remove the link material in a highly localized region without imparting damage to the materials adjacent to, below, or above the target. Selectively processing a designated link may be achieved by varying the laser beam wavelength, spot size, pulse repetition rate, pulse shape, or other spatial or temporal beam parameters that influence energy delivery.
Laser micromachining processes that entail post-processing of conductive links in memory arrays or other types of IC chips use sharp pulses with a fast rising front edge (e.g., with a 1-2 ns rise time) to achieve desired quality, yield, and reliability. To cleanly sever a conductive link, the laser pulse penetrates the overlying passivation layer before cutting through the metal interconnect. The rising edge of a typical pulse from an existing solid state laser varies with pulse width. Use of a traditional Gaussian-shaped laser pulse having a 5-20 ns pulse width and a sloped, gradually rising front edge in link processing tends to cause an “over crater” in the passivation layer, especially if its thickness is too large or is uneven.
Rupture behavior of overlying passivation layers has been well analyzed by Yunlong Sun in his PhD dissertation entitled, “Laser processing optimization of semiconductor based devices” (Oregon Graduate Institute, 1997). Because passivation layer thickness is an important parameter, the optimal thickness of a particular passivation layer material may be determined by simulations based on Sun's analysis. Difficulty in maintaining wafer-level process control of the passivation layer during IC fabrication may result in non-optimal thickness and poor cross-wafer or wafer-to-wafer thickness uniformity. Therefore, optimizing characteristics of laser pulses used in post-processing may help to compensate for mis-targeted dimensions and sources of variation in the passivation layer.
U.S. Pat. No. 6,281,471 of Smart proposes using substantially square-shaped laser pulses for link processing. Such a sharp-edged pulse may be generated by coupling a master oscillator laser with a fiber amplifier (MOPA). This low power master oscillator employs a diode laser that is capable of generating a square-shaped pulse with a fast rise time. On the other hand, in U.S. Pat. No. 7,348,516 of Yunlong Sun et al., which patent is assigned to the assignee of this patent application, states that, despite a vertical rising edge, a substantially square-shaped laser pulse is not the best laser pulse shape for link processing. Instead, Sun, et al. describes use of a specially tailored laser pulse shape that, in one embodiment, resembles a chair, with a fast rising peak or multiple peaks to most effectively process links, followed by a drop-off in signal strength that remains relatively flat at a lower power level before shutting off. Such a tailored laser pulse, with high peak power but low average power, has been successfully generated by what is called pulse slicing technology, which can be implemented by either electro-optical modulation (EOM) or acousto-optical modulation (AOM). For example, a conventional active Q-switched solid state laser provides nanosecond seed pulses with high intensity and high pulse energy, and then a light-loop slicing device transforms a standard laser pulse into a desired tailored pulse shape.
It is possible to obtain high efficiency and high peak power output directly from laser diodes. In other words, it is possible to generate high peak power and high pulse energy using exclusively electrically modulated seed pulses. The simplicity of this scheme is advantageous, and it also may be implemented with fewer amplifier stages. However, the center wavelength of the semiconductor diode tends to drift with small changes in temperature resulting from the change in pulse shape, which temperature drift may adversely affect downstream solid-state amplifiers and harmonic generation.
Alternatively, a specially tailored laser pulse may be generated by a MOPA that employs a gain fiber as the power amplifier. Using a MOPA is advantageous in that it constitutes a stable signal source at a specified constant frequency.
U.S. Patent Application No. 2006/0159138 of Pascal Deladurantaye describes a shaped-pulse laser in which two modulators shape a continuous wave (CW) light beam to generate various shaped pulses. However, generating a pulsed laser from a CW light beam is fairly inefficient, and thus requires more amplification. Because such a low peak-power signal may be influenced by noise, which causes pulse-to-pulse instability, the two modulators are preferably synchronized to maintain pulse stability and energy stability, thereby adding further complexity and cost.